Major shifts in market trends and market share have occurred in the last ten years, with the emergence of "thin and light" Rx spectacle lenses. "Thin" is important to having an attractive appearance for the spectacle wearer (specifically avoiding the undesirable "coke bottle lens" image of wearers of high minus power/demagnifying/myopic prescription lenses for correction of their near-sightedness). "Light" is important because the average spectacle frame places two-thirds of the combined weight of the frame plus lens onto the bridge of the nose, and the greater the weight, the more uncomfortable to the wearer.
This trend to "thin and light" is specifically why in the U.S., glass Rx spectacle lenses now constitute less than 20% market share, and the fastest growing segment is polycarbonate thermoplastic injection molded lens, at center thicknesses ranging from 1.0-1.5 mm, when measured at the optical center in a high minus power lens (in contrast to standard "hard resin" thermoset cast CR-39 lens which cannot safely pass FDA lens safety impact tests at thicknesses much less than 2.0 mm). Because of consumer awareness of "thin and light" benefits, lens product advertising and promotion now prominently features the lens' thickness, so accordingly manufacturing QC practices must accurately measure and control the lens thickness, most commonly now to a plus or minus 0.1 mm tolerance from the nominal advertised value. (Only a dozen years ago, it was not possible to buy Rx FSV thinner than 2.0 mm in U.S.A.).
However, to do this desirable control of lens thickness while managing a production schedule of some 300 or more different combinations of magnifying or demagnifying power with astigmatic power (employing, in turn, mold cavities of hundreds of different convex and concave optically polished mold inserts) has turned out to be a major problem. Typically, such convex and concave lens mold inserts were originally made to a very precise dimensional tolerance themselves. Then, pairing up 1 each of these convex and concave lens mold inserts within each injection mold cavity could thereby assure that the resulting injection molded thermoplastic Rx spectacle lens' thickness will be within dimensional tolerances. However, as a matter of practical operation, when these optically polished mold inserts become scratched or flawed in their optical polished surfaces, the common practice of abrasively lapping/repolishing such scratches removes metal from the original insert, thus reducing its thickness or height dimension. Since such repairs are common and inevitable in the current practice, due to handling damage putting the inserts into and out of the moldsets to change the optical powers of the resulting molded lenses, soon the original inventory of optically polished mold inserts has randomly changed in their thicknesses.
Refer to FIG. 5, "Comparative Example", which shows in cross-sectional view a "prior art" lens mold cavity. Most commonly, the answer has been to place a shim (#27) behind each insert to compensate for its thickness reduction vs. original dimensions. Thus, each time a mold cavity is assembled becomes 1 or more iterations of finding the right combination of shims to assemble with any given pair of convex & concave inserts, to make the lens thickness and power to come out right. Alternatively, thick plating can be put onto either front or back faces of the insert to compensate for its thickness reduction from repair work, then re-grind to size. Both "plating up/regrinding" and assembly with shims have cost and convenience disadvantages which are overcome by the present invention.
Even if somehow all original insert thicknesses met the nominal tolerances and even if there never was any repair or alteration of those dimensions, the conventional means for injection compression molding of such Rx spectacle lenses, with few exceptions, have any provision for reproducible control and predictible settings to assure that the resulting lens thicknesses will stay within the required tolerance of the advertised nominal thickness (most specifically, to be able to hold plus or minus 0.1 mm thickness variation from nominal values, over the full range of differing optical powered lenses to be molded in such a moldset).